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Acoustic metamaterials and metasurfaces have been explored in the past few years to realize a wide range of extreme responses for sound waves. As one remarkable phenomenon, extreme anisotropy and hyperbolic sound propagation are particularly challenging to realize compared to electromagnetic waves because of the scalar nature of airborne acoustics. In parallel, moiré superlattices and the rapidly expanding domain of twistronics have shown that large anisotropy combined with tailored geometrical rotations can enable tantalizing emerging phenomena, such as tailored phase transitions in metamaterials. Connecting these areas of research, here, we explore the realization of acoustic hyperbolic metasurfaces and their combination to drive topological phase transitions from hyperbolic to elliptic sound propagation. The transition point occurring at a specific rotation angle between two acoustic metasurfaces supports highly directional canalization of sound, opening exciting opportunities for twisted acoustics metasurfaces for robust surface wave guiding and steering. [ABSTRACT FROM AUTHOR]

We explore the exceptional point (EP) induced phase transition and amplitude/phase modulation in thermal diffusion systems. We start from the asymmetric coupling double-channel model, where the temperature field is unbalanced in the amplitude and locked in the symmetric phase. By extending into the one-dimensional tight-binding non-Hermitian lattice, we study the convection-driven phase locking and the asymmetric-couplinginduced diffusive skin effect with the high-order EPs in static systems. Combining convection and asymmetric couplings, we further show the phase-locking diffusive skin effect. Our work reveals the mechanism of controlling both the amplitude and phase of temperature fields in thermal coupling systems and has potential applications in non-Hermitian topology in thermal diffusion. [ABSTRACT FROM AUTHOR]

Thermal camouflage utilizes the manipulation of heat fluxes to conceal an arbitrary object in various environments from being detected via thermography. In the past decade, the field of thermal metamaterials and the technique of 3D printing have been rapidly developed, which makes nonintuitive heat flux manipulation feasible. However, when thermal metamaterials are applied to the thermal camouflaging, their conductivities are dependent on the properties of background, leading to the damage of background integrality. Moreover, previous thermal camouflaging schemes have mostly worked in the 2D regime, largely restricting their functional angles and application scenarios, especially in complex environments. Here, wide‐angle radiative thermal camouflaging is realized by using a 3D‐printed meta‐helmet of extremely anisotropic thermal conductivities. Based on 3D coordinate transformation, this meta‐helmet directly maps temperature distributions from the background to the metamaterial surface without damaging background integrity. The non‐invasive device is efficient in wide‐angle thermal camouflage by rendering the same emissivity to the background medium and can self‐adjust to various even unknown background thermal fields, which is demonstrated in numerical simulations and experiments. This work opens a door to the 3D transformation‐thermotics‐based devices for versatile practical applications in thermal infrared stealth of macro‐sized objects and others. [ABSTRACT FROM AUTHOR]

Ever since the Victorian era, montage, the process of pictorial composition made by juxtaposing or superimposing photographs, has been a very popular post-editing imaging technique. Despite showing a strong power in demonstrating complex wave field effects, this technique has neither been fully explored in acoustic imaging nor been realized in real-time systems with the capability beyond diffraction limits. On the other hand, the recent prospect of metamaterials has shown their great potentials in super-resolution acoustic imaging. However, the miracle jigsaw of more advanced functional modulation of acoustic wave fields at deep subwavelength scale still remains elusive. Here we report the experimental implementation of super-resolution acoustic image montage through a judiciously designed biaxial metamaterial lens. Based on the non-diffraction birefringence in the biaxial metamaterials, we realized various montage functionalities such as duplication, composition, and decomposition of sound images with distinctive deep subwavelength features. Our work represents an important step in developing versatile functional acoustic metamaterial devices for imaging purposes, as it provides on-demand editing of sound field patterns beyond diffraction limits. [ABSTRACT FROM AUTHOR]

Various concepts related to parity-time symmetry, including anti–parity-time symmetry, have found broad applications in wave physics. Wave systems are fundamentally described by Hermitian operators, whereas their unusual properties are introduced by incorporation of gain and loss. We propose that the related physics need not be restricted to wave dynamics, and we consider systems described by diffusive dynamics. We study the heat transfer in two countermoving media and show that this system exhibits anti–parity-time symmetry. The spontaneous symmetry breaking results in a phase transition from motionless temperature profiles, despite the mechanical motion of the background, to moving temperature profiles. Our results extend the concepts of parity-time symmetry beyond wave physics and may offer opportunities to manipulate heat and mass transport. [ABSTRACT FROM AUTHOR]

The topological sound transport, as an interesting phenomenon discovered in sonic crystals, has drawn tremendous interest in recent years. Here, in resonant acoustic systems, we demonstrate the existence of band inversion by slightly changing the lengths of tube resonators, which unveils the acoustic valley Hall phase transition characterized by the inverted valley Chern number. However, when the valley topological insulator is largely distorted, we can obtain flat-band-like edge states in the bandgaps with topological protection still existing. Those edge states can propagate along zigzag delay-lines with the backscatterings suppressed to a large amount. Our work provides a prototype of topological-insulator-based acoustic devices with the frequency-selecting functionality. [ABSTRACT FROM AUTHOR]

Topological manipulation of sound has recently been a hot spot in acoustics due to the fascinating property of defect immune transport. To the best of our knowledge, the studies on one-dimensional (1D) topological acoustic systems hitherto mainly focus on the case of the Su-Schrieffer-Heeger model. Here, we show that topologically protected bound states may also exist in 1D periodically modulated acoustic waveguide systems, viz., 1D Floquet topological insulators. The results show that tuning the coupling strength in a waveguide lattice could trigger topological phase transition, which gives rise to topologically protected interface states as we put together two waveguide lattices featured with different topological phases or winding numbers. However, for the combined lattice, input at the waveguides other than the interfacial ones will excite bulk states. We have further verified the robustness of interface bound states against the variation of coupling strengths between the two distinct waveguide lattices. This work extends the scope of topological acoustics and may promote potential applications for acoustic devices with topological functionalities. [ABSTRACT FROM AUTHOR]

[Display omitted] The paradigm shift of Hermitian systems into the non-Hermitian regime profoundly modifies inherent property of the topological systems, leading to various unprecedented effects such as the non-Hermitian skin effect (NHSE). In the past decade, the NHSE has been demonstrated in quantum, optical and acoustic systems. Beside those wave systems, the NHSE in diffusive systems has not yet been observed, despite recent abundant advances in the study of topological thermal diffusion. In this work, we design a thermal diffusion lattice based on a modified Su-Schrieffer-Heeger model and demonstrate the diffusive NHSE. In the proposed model, the asymmetric temperature field coupling inside each unit cell can be judiciously realized by appropriate configurations of structural parameters. We find that the temperature fields trend to concentrate toward the target boundary which is robust against initial excitation conditions. We thus experimentally demonstrated the NHSE in thermal diffusion and verified its robustness against various defects. Our work provides a platform for exploration of non-Hermitian physics in the diffusive systems, which has important applications in efficient heat collection, highly sensitive thermal sensing and others. [ABSTRACT FROM AUTHOR]

Non-Hermitian wave system has attracted intense attentions in the past decade since it reveals interesting physics and generates various counterintuitive effects. However, in the diffusive system that is inherently non-Hermitian with natural dissipation, the robust control of heat flow is hitherto still a challenge. Here we introduce the skin effect into diffusive systems. Different from the skin effect in wave systems, where asymmetric couplings were enabled by dynamic modulations or judicious gain/loss engineering, asymmetric couplings of the temperature fields in diffusive systems can be realized by directly contacted metamaterial channels. Topological heat funneling is further presented, where the temperature field automatically concentrates towards a designated position and shows a strong immunity against the defects. Our work indicates that the diffusive system can provide a distinctive platform for exploring non-Hermitian physics as well as thermal topology. Non-Hermitian systems can house a range of unusual physical phenomena such as the skin-effect which has been typically observed in optical lattices and electrical circuits. Here, the authors show the non-Hermitian skin effect in thermal transport, demonstrating that the topologically protected heat flow can be realized by using thermal metamaterials. [ABSTRACT FROM AUTHOR]

We investigate analytically, numerically, and experimentally the low-loss supermode propagation in a coupled acoustic waveguide complex within a broadband. The waveguide complex is implemented with air channels coupled via an ultrathin metafluid layer. We analytically derive the field distribution of incident sound needed for producing acoustic supermodes, and verify the periodically revival propagation in coupled waveguide systems numerically and experimentally. We find out that the supermode wavelength becomes longer for higher mode order or lower frequency. We have also demonstrated the robust propagation of supermodes in broadband. Our scheme can in principle be extended to three dimensions and the ultrasound regime with simplicity and may promote applications of high-fidelity signal transfer in complicated acoustic networks. [ABSTRACT FROM AUTHOR]

Valley pseudo-spin and its associated interface wave transport in sonic crystals has attracted increasing attention from researchers for the potential manipulation of acoustic waves. The topological interface state, projected from a specific valley, is valley-locked, and, thus, renders robust reflection immunity against defects. In this work, we report on the experimental observation of the different parity generations of interface states at two distinct zigzag interfaces. By designing a "C"-shaped domain wall, we experimentally demonstrated the parity generation and selective excitation of interface valley-locked states. Benefiting from different parities of the interface states, one-way valley parity conversion was verified in sonic crystals without breaking the time reversal symmetry. Our findings contribute to the applications in noise control, acoustic communication, and logic processing for topological functional devices with unidirectional responses. [ABSTRACT FROM AUTHOR]

Metasurfaces of the subwavelength thicknesses provide a distinctive route for acoustic wave manipulation. Based on the advanced 3D printing, those judiciously designed 2D metamaterials enable intriguing effects such as abnormal reflection, transmission, and absorption. However, acoustic metasurfaces, with strong wave–structure interactions in subwavelength scales, have encountered a big challenge of being acoustically transparent due to the insufficient impedance mismatch underwater. Here, reconfigurable bioinspired metaskin patterning for generating multistructured waterborne ultrasound is proposed. The nanostructured metaskin exhibits the "lotus effect," with the thickness of only 70 μm (≈1/20 wavelength at 1 MHz) and a tremendous impedance mismatch (≈0.0001 transmission) for ultrasound. By depositing the strippable metaskins via self‐assembly into different patterns, the focusing, the vortex, and the Talbot structured ultrasound beams are implemented, respectively. The multistructured ultrasound has the patterned intensity fields of energy redistribution, where even weak field enhancement at low frequencies can activate the living organisms directly without using microbubbles, which enables low‐threshold and contactless behavior control via the mechanical stimulus. [ABSTRACT FROM AUTHOR]

Breaking the diffraction limit to achieve super-resolution focusing is a long-sought goal in the field of acoustic imaging and detection. Here, we demonstrated super-resolution focusing of high-frequency ultrasound (5 MHz) based on an acoustic meta-lens with four centrosymmetric dart-like sharp-edge apertures. For the diffracted ultrasound carrying high spatial frequencies, super-resolution focusing is generated in the far field with mainlobe size breaking the diffraction limit (<1.22λ). Different from previous far-field super-resolution focusing, where the sidelobe intensity is close to or even larger than the mainlobe intensity, the sidelobe intensity in our case is weak. We further conducted fine defect detection (∼200 μm) by utilizing our designed far-field super-resolution focusing to verify its nondestructive testing performance. [ABSTRACT FROM AUTHOR]

• The twisted bilayer gratings are proposed for tunable guided resonances based on the acoustic moiré effect. • The transmission spectra are obtained by the Fast Fourier Transform analysis for different twisting angles. • This work extends the moiré metasurface from optics into acoustics from the experimental perspective. Recently, the moiré pattern has attracted lots of attention by superimposing two planar structures of regular geometries, such as two sets of metasurfaces or gratings. Here, we show the experimental investigation of acoustic moiré effect by using twisted bilayer gratings (i.e., one grating twisted with respect to the other). We observed the guided resonance that occurred when the incident ultrasound beam was coupled with the guiding modes in a meta -grating, significantly influencing the reflection and transmission. Tunable guided resonances from the moiré effect with complete ultrasound reflection at different frequencies were further demonstrated in experiments. Combining the measurements of transmission spectra and the Fast Fourier Transform analyses, we reveal the guided resonance frequencies of moiré ultrasonic metasurface can be effectively controlled by adjusting the twisting angle of the bilayer gratings. Our results can be explained in a simplified model based on the band folding theory, providing a reliable prediction on the precise control of ultrasound reflection via the twisting angle adjustment. Our work extends the moiré metasurface from optics into acoustics, which shows more possibilities for the ultrasound beam engineering from the moiré effect and enables the exploration of functional acoustic devices for ultrasound imaging, treatment and diagnosis. [ABSTRACT FROM AUTHOR]

We propose a design of tunable double-band perfect absorbers based on the resonance absorption in acoustic metasurfaces with nesting helical tracks and deep-subwavelength thicknesses (<λ/30 with λ being the operation wavelength). By rotating the cover cap with an open aperture on the nesting helical tracks, we can tailor the effective lengths of resonant tubular cavities in the absorber at will, while the absorption peak frequency is flexibly shifted in spectrum and the acoustic impedance is roughly matched with air. The simulated particle velocity fields at different configurations reveal that sound absorption mainly occurs at the open aperture. Our experiment measurements agree well with the theoretical analysis and simulation, demonstrating the wide-spectrum and tunable absorption performance of the designed flat acoustic device. [ABSTRACT FROM AUTHOR]

Keywords: bioinspired metaskins; lotus effect; mechanical stimuli; self-assembled nanosurfaces; structured ultrasound EN bioinspired metaskins lotus effect mechanical stimuli self-assembled nanosurfaces structured ultrasound 1 1 1 08/18/22 20220815 NES 220815 B Bioinspired Metaskin Patterning b In article number 2203109, Long Meng, Hairong Zheng, Xue-Feng Zhu, and co-workers report a reconfigurable acoustic meta-skin with selective superhydrophobic decoration on a metal surface, which can be utilized for generating different structured ultrasounds and have potential applications in biological low-intensity ultrasound stimulation of living organisms for the study of behavioral response and contactless control. Generating Multistructured Ultrasound via Bioinspired Metaskin Patterning for Low-Threshold and Contactless Control of Living Organisms (Adv. Funct. Bioinspired metaskins, lotus effect, mechanical stimuli, self-assembled nanosurfaces, structured ultrasound. [Extracted from the article]